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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 04-08-2010, 04:22 PM
roltex_rohit123 roltex_rohit123 is offline
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Default testing transistors and capacitors.

how can i test a transistor like hsd965, a power mosfet like IRF9z34 and general NPN and PNP transistor with my multimeter? also i wanted to know how to test capacitors.
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Old 04-08-2010, 05:09 PM
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For transistors, a diode check function is most handy. Most meters have that included.

In general, you should see a PN junction Base to Emitter, and Base to Collector. Unless the transistor is germanium, there should not be any conduction Emitter to Collector in any polarity.

For FET's, there is the inherent Drain to Source diode, which is in opposition to the polarity of the device. If you can read that PN junction, and all other terminals show open, the FET is probably good. What is handy is that they nearly fail short, whereas transistors can go open or short.

Capacitors other than electrolytic don't fail without changing color or showing obvious signs. Electrolytics often show swollen cases, split vents, or do explode. If your capacitor is in one piece, it will be within the stated tolerance. We are done with the old wax and foil and silver mica monsters that did show value changes.
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Old 04-08-2010, 05:17 PM
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Sometimes, capacitors fail without showing any external signs of distress.

Just the other day, I repaired one of our televisions that had a failed 220uF 200v electrolytic capacitor. One of the leads had separated internally from it's plate connection, resulting in an open circuit. The TV's picture had displayed obvious signs of a cap failure in the power supply (bad 60Hz "waves" on screen) before it's supply resistor burned up.

If your meter is the old fashioned D'arsonval type meter movement, you can discharge the capacitor with a low value resistor, then set your meter to read Ohms, and touch the test leads to the terminals. The meter needle will momentarily go towards nearly 0 Ohms, and then settle towards infinite.

A safer method is to discharge the capacitor, and then charge it via a resistor while measuring the voltage across the cap. You can then get an idea of the capacitance via the RC time constant.

See this chapter in our E-book: http://www.allaboutcircuits.com/vol_..._16/index.html
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Last edited by SgtWookie; 04-08-2010 at 05:58 PM.
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Old 04-08-2010, 06:57 PM
roltex_rohit123 roltex_rohit123 is offline
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Quote:
Originally Posted by SgtWookie View Post
Sometimes, capacitors fail without showing any external signs of distress.

Just the other day, I repaired one of our televisions that had a failed 220uF 200v electrolytic capacitor. One of the leads had separated internally from it's plate connection, resulting in an open circuit. The TV's picture had displayed obvious signs of a cap failure in the power supply (bad 60Hz "waves" on screen) before it's supply resistor burned up.

If your meter is the old fashioned D'arsonval type meter movement, you can discharge the capacitor with a low value resistor, then set your meter to read Ohms, and touch the test leads to the terminals. The meter needle will momentarily go towards nearly 0 Ohms, and then settle towards infinite.

A safer method is to discharge the capacitor, and then charge it via a resistor while measuring the voltage across the cap. You can then get an idea of the capacitance via the RC time constant.

See this chapter in our E-book: http://www.allaboutcircuits.com/vol_..._16/index.html
hey i'm a newbie and moreover i am a medical biotechnologist just started to leanr electronics. have some pity on me...
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Old 04-08-2010, 07:10 PM
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Gee, I thought I was being helpful, and even gave you a reference where you can learn about RC time constants.

If you don't understand a portion of what was written, please quote the portion that you don't understand, and we will try to explain it so that it is more easily understood.
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Old 04-08-2010, 07:13 PM
roltex_rohit123 roltex_rohit123 is offline
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Quote:
Originally Posted by SgtWookie View Post
Gee, I thought I was being helpful, and even gave you a reference where you can learn about RC time constants.

If you don't understand a portion of what was written, please quote the portion that you don't understand, and we will try to explain it so that it is more easily understood.
yes i will do all to be good at electronics.. thankyou
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